Primates are an order of mammals made up of prosimians, tarsiers, Old and New World monkeys, and simians (apes). Primates are not closely related to any other mammalian order. The most closely related species to primates are shrews (Scandentia) and colugos (Galeopithecus), arboreal gliding mammals native to Southeast Asia. The oldest primate fossil dates back to the late Cretaceous, around 60 million years ago. Scientists believe that the common ancestor of all primates lived around 81 million years ago, before the extinction of the dinosaurs. Older ancestors dating back 100 million years resembled shrews, with four short legs, a long tail, eyes positioned on the sides of the head, and a small cerebral cortex.

Primates have several defining characteristics: an opposable thumb for grasping objects, flat nails, color vision, two mammary glands in the chest, and both eyes facing forward on the face.

Simians, or apes, evolved from Old World monkeys 24 to 33 million years ago. The main characteristic of simians is that they do not have tails. Few fossils from this period have been discovered. Two include Aegyptopithecus zeuxis, which displayed ape-like features, and the ape, Proconsul africanus, a common ancestor of both humans and African apes.

While the evolution of primates is not entirely understood, it is known that they reached all of Earth's continents, except for Australia and Antarctica. Apes live in Asia and Africa, and monkeys can be found in Asia, Africa, and the Americas. Monkey fossils have also been found in Europe.

Tracing primate evolution is not an easy task. We must consider all of the anatomical, molecular, and genetic similarities of this group, along with the geological changes of the last 60 million years. A recent study incorporating the location of modern-day species, the primate evolutionary tree, and the location of existing fossils proposes that Old World monkeys, apes, and humans evolved in Africa and Eurasia in the following manner:

• The ancestors of Old World monkeys and apes, contemporaries of Aegyptopithecus zeuxis, first appeared in Africa.

• Proconsul africanus, and other related species, evolved in Africa.

• Migration into Europe and Asia took place around 20 million years ago.

• The ancestors of some extinct hominid species and today's Asian apes, the orangutans and gibbons, evolved in Eurasia. One such ancestor is Oreopithecus bambolii.

• Ten million years ago, a population of apes returned to Africa. They evolved into today's African apes, the chimpanzees, bonobos, gorillas, and our human ancestors, the hominins.

Old World monkeys have round nostrils that point downward and do not have prehensile tails. (A prehensile limb can be used for grasping). In contrast, New World monkeys have long, prehensile tails, flattened snouts, and side-pointing nostrils. They all share a common ancestor. How are these Old World and New World monkeys related to one another?

The oldest known primate fossils in the Americas are 23 million years old, while those from Africa, as we have already mentioned, are 60 million years old. Hence, the New World monkeys evolved later.

Based on the genetic studies of monkeys on both sides of the Atlantic, a consensus now exists that New World monkeys share a common ancestor with Old World monkeys and apes. This common ancestor lived about 40 million years ago. The next question is: How did the monkeys reach America? Africa separated from South America about 140 million years ago as the South Atlantic Ocean opened. There were no primates on Earth 140 million years ago. Therefore, monkeys could not have arrived in South America as a result of continental drift.

Monkeys may have arrived in South America from Africa on makeshift rafts formed by plant debris during tropical storms. Currently, the shortest distance between Brazil and Africa is 2,900 km. However, 40 million years ago, the shortest distance was 1,500 km. In addition, there is evidence that the depth of the Atlantic Ocean has deepened over time. Therefore, there may have been islands between the continents. If we factor in that oceanic currents move from east to west, it is not unreasonable to estimate that a trans-Atlantic trip 40 million years ago could have taken between 7 and 11 days.

The evolution of apes is particularly interesting because they are our closest living relatives on Earth. It's important to note that although we share a recent common ancestor with the apes, our behaviors can be quite diverse.